[identity][websocket] Call DB client directly to verify CSAT

Summary:
Noticed that we're doing an unnecessary round-trip with search WebSocket auth:
Identity WS, when received auth message, connected to itself via gRPC just to verify CSAT.

Replaced this with a direct DDB call.

Test Plan: Verified that identity search is still working on local clients (with local Identity).

Reviewers: will, varun

Reviewed By: will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12447